The Dutchman has already scored 14 goals in 20 Premier League appearances for United as they sit seven points clear at the top of the table, and Mancini says that he could be crucial to deciding the title race.
"We were very close to signing him," Mancini told reporters. "I know what the reason was, but I won't say. It wasn't about money.
|VAN THE MAN?
|6/4||Robin van Persie is 6/4 to be the top Premier League goalscorer with bet365|
"We wanted Van Persie because we knew he was different from our other strikers. We wanted to improve our team, particularly for the Champions League.
"Van Persie is a very important player for United. He's scored 14 league goals and it's clear he has changed their situation. At the moment, he is the difference between us and United.
"I don't know if he listened to his heart. He took his decision but three or four months before that we were there."